Sharp-crested weirs: for a clinging (depressed) nappe, the typical excess discharge over a free, ventilated nappe is approximately

Introduction / Context:
Weir calibration assumes a free, ventilated nappe. When air cannot freely access the nappe underside, sub-atmospheric pressure forms, causing the sheet to cling to the downstream face. This “clinging nappe” increases the effective discharge for a given head and invalidates standard coefficients unless corrected.

Given Data / Assumptions:

• Sharp-crested rectangular or triangular weir.
• Comparison between ventilated (free) and non-ventilated (clinging) nappe at the same head.
• Similar approach conditions.

Concept / Approach:

When the nappe clings, pressure on its underside drops below atmospheric. The increased pressure differential accelerates the flow, raising discharge beyond the value predicted for a free nappe by roughly several percent—commonly cited about 6–7% for many practical cases.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory calibrations show consistent increases on the order of a few percent; restoring ventilation returns measured Q to free-nappe coefficients, confirming causality.

Why Other Options Are Wrong:

(b)–(e) greatly overstate the effect for standard weirs and heads; such large increases are not typical and would indicate other errors or geometric changes.

Common Pitfalls:

Failing to provide aeration vents; reading head too close to the crest; using free-nappe coefficients with a clinging nappe without correction.

Final Answer:

6% to 7%